Coated Nanoparticles and Quantum Dots for Solution-Based Fabrication of Photovoltaic Cells

ABSTRACT

CIGS absorber layers fabricated using coated semiconducting nanoparticles and/or quantum dots are disclosed. Core nanoparticles and/or quantum dots containing one or more elements from group 13 and/or IIIA and/or VIA may be coated with one or more layers containing elements group IB, IIIA or VIA. Using nanoparticles with a defined surface area, a layer thickness could be tuned to give the proper stoichiometric ratio, and/or crystal phase, and/or size, and/or shape. The coated nanoparticles could then be placed in a dispersant for use as an ink, paste, or paint. By appropriate coating of the core nanoparticles, the resulting coated nanoparticles can have the desired elements intermixed within the size scale of the nanoparticie, while the phase can be controlled by tuning the stochiomctiy, and the stoichiometry of the coated nanoparticle may be tuned by controlling the thickness of the coating(s).

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/954,183 filed Dec. 11, 2007, which is a divisional of U.S. patentapplication Ser. No. 10/943,657 filed Sep. 18, 2004, now issued as U.S.Pat. No. 7,306,823 on Dec. 11, 2007, all of which are herebyincorporated by reference for all purposes.

FIELD OF THE INVENTION

This invention is related to photovoltaic cells and more particularly tofabrication of IB-IIIAVIA active layers for such cells.

BACKGROUND OF THE INVENTION

Low-cost production of solar cells on flexible substrates using printingor web coating technologies is promising a highly cost-efficientalternative to traditional silicon-based solar cells. Recently, solarcells with absorber layers fabricated by solution-based deposition ofalloys of copper (Cu) and indium (In) with selenium (Se) or sulfur (S)have been developed. Such solar cells, generally referred to as GIGScells, have been fabricated using different non-vacuum processes inwhich a precursor solution is formulated containing mixed oxides of Cu,In and Ga, which is then coated on a substrate. In particular, Kapur etal (U.S. Pat. No. 6,268,014, issued October 2000) describe a method forfabricating a solar cell based upon the solution-based deposition of asource material comprised of mechanically milled, oxide-containing,sub-micron sized particles, while Eberspacher and Pauls (U.S. Pat. No.6,268,014, issued July 2001; US Patent Application Publication20020006470) describe the forming of mixed metal oxide, sub-micron sizedparticles by pyrolizing droplets of a solution, then ultrasonicallyspraying the resulting particles onto a substrate.

However, there are several drawbacks to the use of metal oxides asprecursor materials for GIGS solar cells. First, the use of oxide-basedparticles in CIGS absorber layer construction requires ahigh-temperature hydrogen reduction step to reduce the oxides. Inaddition to requiring substantial time and energy, this step ispotentially explosive. Further, although it is highly desirable toincorporate gallium in the active layer of the solar cells, the presenceof gallium results in the formation of gallium oxide upon annealing, ahighly stable material which is very difficult to reduce even under themost extreme conditions. As a result, it is very difficult toeffectively incorporate gallium into a nascent copper indium precursorfilm using a metal oxide synthesis approach.

In addition, the methods of particle formation and deposition taught inthe prior art early significant challenges. For example, mechanicalmilling is a lengthy process that can requires substantial energy andtake several tens of hours to achieve sub-micron sized particles.Further, even after milling, particles are rarely uniform, resulting ina substantial size distribution, which can result in poorly packedprecursor films, leading to low-density absorber layers with pooroptoelectronic and electronic properties. Spray pyrolysis of micronthick layers of precursor particles also has significant drawbacks.First, the ultrasonic spraying of thin layers of sub-micron sizedparticles onto a substrate is an inherently non-uniform process,resulting in differential drying rates as particles are spray deposited.Non-uniform drying can result from any of several factors including butnot limited to differential drying on the substrate, mid-stream drying(e.g. drying of droplets prior to reaching a substrate), and pooling ofparticles and droplets into non-contiguous aggregates that leave spacebetween the aggregation loci. Further, it is especially challenging toachieve any scaling for this technique since it is inherently difficultto carry out a uniform wet deposition of many small particle-containingdroplets over a large area without any premature drying prior tocompletion of the deposition process. Films are often uneven and havesubstantial spatial non-uniformities across their surface. These andrelated forms of nonuniform drying lead to the formation of pockets andvoids within the deposited film, creating a porous material which leadsto a solar cell with poor and unstable optoelectronic and electronicproperties. Some of these defects can be overcome when much thickerfilms are deposited, e.g. in the 20 to 100 micron thick range, but suchfilms are not useful for solar cell devices, which typically require theabsorber layer to have a thickness between about one and two microns.

Rapid drying of thin films also limits the potential scaleability ofspray deposition techniques. For example, using a scrolling nozzle tospray deposit across a large surface area results in drying of theinitially sprayed area prior to deposition of the final area to bedeposited. This uneven drying results in additional spatialnon-uniformities, pockets, and voids. While the use of multiple sprayheads concurrently moving across a substrate surface decreases the timerequired for deposition across the total surface area, localnon-uniformities can arise from regions near each of the nozzles as wellas in overlapping regions.

For sub-micron precursor particles formed by either mechanical millingor spray pyrolysis, it is difficult to make precisely-shaped and sizednanoparticles of indium and gallium since indium is so soft and galliumis molten at room temperature. Small and uniformly-sized particles leadto more densely-packed films, which can improve device performance.Furthermore, the properties of the resulting CIGS absorber layer arehighly dependent on the stoichiometric ratio of the elements in thelayer. The stoichiometric ratios in the layer largely depend on thestoichiometric ratios in the nanoparticles in the precursor solution,while the initial crystalline phase of precursor materials impacts thefeasibility of achieving a targeted final phase in the annealed absorberfilm. It is difficult to precisely tune the stoichiometry and/or phaseof the nanoparticles on a nanometer scale with current techniques.

Thus, there is a need in the art, for a non-oxide, nanoparticle basedprecursor material that overcomes the above disadvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIGS. 1A-1C are a sequence schematic diagrams illustrating formation ofcoated nanoparticles for a paint, ink, or paste for use in forming anabsorber layer of a photovoltaic device according to an embodiment ofthe present invention.

FIGS. 2A-2C are a sequence schematic diagrams illustrating formation ofcoated nanoparticles for a paint, ink, or paste for use in forming anabsorber layer of a photovoltaic device according to an alternativeembodiment of the present invention.

FIG. 3 is a cross-sectional schematic diagram illustrating an apparatusfor coating nanoparticles by atomic layer deposition according to anembodiment of the present invention.

FIG. 4 is a flow diagram illustrating the fabrication of a photovoltaicdevice according to an embodiment of the present invention.

FIG. 5 is a schematic diagram of a photovoltaic cell according to anembodiment of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Although the following detailed description contains many specificdetails for the purposes of illustration, anyone of ordinary skill inthe art will appreciate that many variations and alterations to thefollowing details are within the scope of the invention.

Accordingly, the exemplary embodiments of the invention described beloware set forth without any loss of generality to, and without imposinglimitations upon, the claimed invention.

The disadvantages associated with the prior art may be overcome byfabricating CIGS absorber layers using coated nanoparticles.Nanoparticles are discrete entities sized less than about 1000 nm, morepreferably less than about 500 nm, and still more preferably less thanabout 250 nm. When the nanoparticles are sized less than about 10 nm,their chemical, physical, electronic, and optical properties oftenchange relative to that of bulk material, and at about the 10 nm or lesssize scale, nanoparticles are also called “quantum dots”.

Various coatings could be deposited, either singly, in multiple layers,or in alternating layers, all of various thicknesses. Specifically, asshown in FIGS. 1A-1C, core nanoparticles 102 containing one or moreelements from group IB and/or IIIA and/or VIA may be coated with one ormore layers 104, 106 containing elements of group IB, IIIA or VIA toform coated nanoparticles. Preferably at least one of the layerscontains an element that is different from one or more of the group IB,IIIA or VIA elements in the core nanoparticle. The group IB, IIIA andVIA elements in the core nanoparticle 102 and layers 104, 106 may be inthe form of pure elemental metals or alloys of two or more metals. Byway of example, and without limitation, the core nanoparticles mayinclude elemental copper, or alloys of copper with gallium, indium, oraluminum and the layers 104 may be gallium, indium or aluminum.

Using nanoparticles with a defined surface area, a layer thickness couldbe tuned to give the proper stoichiometric ratio within the aggregatevolume of the nanoparticle. By appropriate coating of the corenanoparticles, the resulting coated nanoparticles can have the desiredelements intermixed within the size scale of the nanoparticle, while thestoichiometry (and thus the phase) of the coated nanoparticle may betuned by controlling the thickness of the coating(s).

Prior to use in a solution-based formulation, individual nanoparticlesformed by a dry synthesis technique are typically collected as a drynanopowder. For nanoparticles formed by wet synthesis techniques,individual nanoparticles are typically formed in a solvent system wherethey are maintained as individual particles within the solution.

The coated nanoparticles could then be placed in a dispersant for use asa solution-based semiconductor formulation. The formulation can be inthe form of an ink, paint or paste. Ink is defined as a “pigmentedliquid”. In contrast, a paint is defined as “a liquid mixture, usuallyof a solid pigment suspended in a liquid vehicle”. A paste is defined as“any mixture of a soft and malleable consistency”. A pigment is amaterial that selectively absorbs certain regions of the light spectrum.

By way of example, the core nanoparticles may be copper (Cu)nanoparticles. These core nanoparticles may be of any suitable diameter,e.g., between about 0.1 nm and about 1000 nm, more preferably betweenabout 1 nm and about 500 nm, even more preferably between about 1 nm and250 nm, still more preferably between about 1 nm and 100 nm, and mostpreferably between about 1 nm and 20 nm. If desired, the sizedistribution of the corenanoparticles may be narrowed, e.g., byultrafiltration. In addition to nanoparticles, nanowires, nanorods,nanobelts, nanowiskers, and other shapes can be used for the corenanoparticles. Other nanoparticles can also be coated, e.g., a particlecore comprised of metals other than Cu, or comprised of an organicmaterial such as polystyrene.

The core nanoparticles may be obtained by purchase of suitablecommercially available nanoparticle materials. Alternatively the corenanoparticles may be formed by a suitable method. Several methods areavailable for the formation of nanoparticles, including but not limitedto dry-synthesis techniques such as evaporation-condensation (Granqvistand Buhrman, 1976, “Ultrafine metal particles”, J. Applied Physics 47(5): 220-2219); and the electro-explosion of wire (Tepper, “NanosizedAlumina Fibers”, Advanced Materials, American Ceramic Society Bulletin,Vol. 80, No. 6, June 2001), and wet chemical techniques such asorganometallic synthesis of colloids (Hambrock et al., “Nano-Brass:Bimetallic Copper/Zinc Colloids by a Nonacqueous Organometallic Routeusing [Cu(OCH(Me)CH2NMes)₂] and Et2Zn as Precursors” Chem. Mater. 15:4217-4222, (2003)), metal-salt reduction (E. E. Carpenter., C. T. Seip,C. J. O'Connor, J. Appl. Phys., 85, 5164 (1999)), redox transmetalationreaction in warm toluene solution (J. Park, J. Cheon, J. Am. Chem. Soc.,123, 5743 (2001)), the creation of nanoparticles by electrochemistry(e.g. anodization in an oxygen-free, sonicated environment containing asurfactant) (see Chem. Fur. J. 2001, 7, 1084), and a combination of hightemperature decomposition of a metal carbonyl precursor and thereduction of a metal salt in the presence of surfactants (S. Sun, C. B.Murray, D. Weller, L. Folks, A. Moser, Science, 287, 1989 (2000)).

As used herein, “obtaining” core nanoparticles includes both purchase ofthe core nanoparticles and formation of the core nanoparticles. Forexample, the core nanoparticles 102 may be formed by electro-explosionof copper wires. Cu nanopowders can also be obtained commercially. Forexample, Cu nanoparticles are available commercially, e.g., fromMaterials Modification Inc., of Fairfax, Va. Layer 104 may be a layer ofgallium (Ga) or indium (In). Layer 106 may be a layer of aluminum and/orindium.

There are many variations on the embodiment described above, forexample, the core nanoparticles 102 may be made of indium and the layers104, 106 may include copper, and/or gallium and/or aluminum.Alternatively, the core nanoparticles 102 may be aluminum and the layers104, 106 may include copper, and/or gallium and/or indium. Since,gallium is liquid at room temperature, it is usually undesirable for theouter layer 106 to be gallium metal. Otherwise, the molten gallium willtend to cause the resulting nanoparticles stick together. Thus thecoated particles would likely be unstable under typical manufacturingconditions. The layers 104, 106 may optionally include a layer of agroup VIA element such as selenium (Se) or sulfur (S). Thus, the coatingsteps may produce Cu—In—Ga or Cu—In—Ga(Se, S) nanoparticles for use inan ink, paste, and/or paint for solution deposition of an absorber layerin a photovoltaic cell.

In a variation on the above embodiment, bimetallic core nanoparticlescontaining two or more different elements, e.g., nanoparticles comprisedof solid solutions or alloys of Cu—Ga, Cu—Al, or Cu—In, may be coatedwith one or more layers of metal such as Cu, In, Ga, or Al. Bimetallicnanoparticles may be obtained, e.g., by reducing metal salts ontometallic nanoparticles. In an alternative approach, bi metallicnanoparticles can be formed via evaporation-condensation techniquesusing a bimetallic source, or through co-evaporationcondensationtechniques using more than one source. In addition, wet chemicaltechniques can also be used to form bimetallic nanoparticles through thecombination of more than one metal precursor, e.g. for use in a metalsalt reduction, organometallic colloid synthesis, and/or redoxtransmetalation reaction. Alternatively, bimetallic nanoparticles may beobtained by electro-explosion of bimetallic wires. For example, coreCu—Ga nanoparticles may be obtained by electro-explosion of Cu—Ga wires.In this example, the core nanoparticles 102 may be a solid solution ofcopper and gallium and the layers 104, 106 may include indium and/oraluminum.

In an alternative embodiment, depicted in FIGS. 2A-2C, corenanoparticles may be divided into two portions as shown in FIG. 2A. Asshown in FIG. 2B, first portion 200 core nanoparticles 202 are coatedwith a layer 204 of indium. In a second portion 210 core nanoparticles212 are coated with Ga. The two portions may then be mixed together toform a mixture 220 as shown in FIG. 2C for use in an ink, paint, orpaste.

In examples described above, core nanoparticles containing Cu may becoated with a layer of In or Ga or both. if desired, the nanoparticlesmay be further coated with additional layers, including one or morelayers containing group VIA elements, e.g., Se or S. It should also beunderstood that group TB, IIIA, and VIA elements other than Cu, In, Ga,Sc, and S may be use in the coated nanoparticles described herein, andthat the use of a hyphen (“-” e.g., in Cu—Se or Cu—In—Se) does notindicate a compound, but rather indicates a coexisting mixture of theelements joined by the hyphen. Where several elements can be combinedwith or substituted for each other, such as In and Ga, or Se, and S, inembodiments of the present invention, it is not uncommon in this art toinclude in a set of parentheses those elements that can be combined orinterchanged, such as (In, Ga) or (Se, S). The descriptions in thisspecification sometimes use this convenience. Finally, also forconvenience, the elements are discussed with their commonly acceptedchemical symbols. Group IB elements suitable for use in the method ofthis invention include copper (Cu), silver (Ag), and gold (Au).Preferably the group IB element is copper (Cu). Group IIIA elementssuitable for use in the method of this invention include gallium (Ga),indium (In), aluminum (Al), and thallium (T1). Preferably the group IIIAelement is gallium (Ga) and/or aluminum (Al) and/or indium (In). GroupVIA elements of interest include selenium (Se), sulfur (S), andtellurium (Te), and preferably the group VIA element is either Sc or S,or both Se and S.

Coating of Nanoparticles

There are a number of different ways of coating the nanoparticles. Byway of example, the core nanoparticles (e.g., copper) may be placed inan indium-containing or gallium-containing or aluminum-containingchemical bath for defined period of time. The chemical bath can becooled to prevent melting of the indium or gallium during coating, orheated to cause the controlled decomposition of heat-sensitive precursormaterials. Alternatively, the core nanoparticles may be coated by atomiclayer deposition (ALD). The metal layers can be deposited in a fairlyconformal, nearly uniform fashion through either chemical bathdeposition, electroless plating, or atomic layer deposition, orcombinations of these and/or similar techniques, with the first twodeposition methods typically taking place in a liquid environment, andthe last deposition technique typically taking place in a gasenvironment.

For chemical bath deposition onto copper nanoparticles, the thickness ofthe indium or gallium layer is set by the particle residence time in thechemical bath. For atomic layer deposition, the thickness of the coatinglayer is set by a range of variables including but not limited to thetotal number of deposition cycles and the substrate temperature.

By way of example and without loss of generality, Cu or Cu/Gananopowders can be dispersed in an aqueous solution and then coated withindium and/or gallium and/or aluminum metal using one of the followingelectroless plating processes.

1. Alkaline Indium Electroless Plating Bath

In a first example an electroless indium plating bath under alkaline orweakly alkaline conditions may contain hydrazine or sodium borohydrideas a reducing agent. An aqueous solution of Indium salts may be preparedwith Indium salts can be Indium (I) sulfate, Indium(III) sulfate, Indium(I) nitrate, Indium(III) nitrate, Indium (I) chloride, Indium(III)chloride, Indium (I) Iodide, Indium(III) iodide, Indium (I) trifiate(trifluoromethanesulfonate), Indium(III) trif7ate(trifluoronlethanesulfonate), Indium (I) acetate, or Indium(III) acetateat a concentration of 1 to 30 mM, preferably 5 to 10 mM. Water used inthe plating bath is distilled and deionized with a resistance more than1 Mega Ohms, preferably more than 10 Mega Ohms. Chelatinb, capping,complexing agents or stabilizers Such as sodium citrate or potassiumcitrate or nitrilotriacetic acid, trlsodium salt, orethylenediaminetetraacetic acid, disodium salt, trictha.nol'.mine,pyridine, polyvinylpyrilidinol (PVP), trioctylamines, TOAB, and/orsimilar compounds may be added into the above indium salt solution. Theconcentration of the chelating agents or surfactants may range from 10to 100 mM, preferably from 20 to 60 mM. A reducing agent, (such ashydrazine or sodium born-hydride) is slowly added into the mixture andthe final concentration of the reducing agent may range from 20 to 200mM, preferably from 50 to 100 mM. The pH value for the final solution isadjusted to between 8 and 12, preferably in the range of 9 to 11. Thetemperature of the plating bath is raised to between 30° C. and 90° C.,preferably between 40° C. and 60° C. Precise control of the ramping rateand the final temperature allow for precise control of the depositionrate of indium. In addition to temperature, the pH value, concentrationof indium in the solution, and the plating time also determine the finalthickness of the indium layer that is deposited. The plating time mayvary from 10 to 300 minutes. The thickness of the indium layer alsodepends on the particle size of the copper nanopowders. For copper orcopper-gallium nanopowders with an average particle size of 20 nm, theindium layer may be made to the range of 10 to 50 nm. The thickness ofthe indium layer is related to the residence time of the nanoparticlesin the chemical bath. The suspension of coated Cu or Cu/Ga nanoparticlescan be directly used as coating media for a CIGS absorber layer, orcoated with a second and/or a third shell layer of Al and/or Ga and/orIn as described herein, then used as a coating media for a solar cellabsorber layer. Alternatively, the suspension can be diluted orconcentrated to form a nanoparticle ink, paint, or paste formulationwith a desired viscosity and/or dispersivity. Techniques such ascentrifugation and filtration can also be used to collect coatednanoparticles, either singly or in combination. Then the coatednanoparticles can be redispersed in a dispersion/suspension system toform an ink, paint or paste.

2. Acidic Indium Electroless Plating Bath

In a second example an acidic indium electroless plating bath mayinclude indium salts and thiourea in an aqueous solution with a low pHvalue. Chelating agents may be used to stabilize indium ions ifnecessary for a conformal coating on copper nanopowders. Unlike alkalineindium electroless plating, a reducing agent is not required. Indiumsalts can be Indium (I) sulfate, Indium(III) sulfate, Indium (I)nitrate, Indium(III) nitrate, Indium (I) chloride, Indium(III) chloride,Indium (I) Iodide, Indium(III) iodide, Indium (I) triflate(trifluoromethanesulfonate), Indium(III) triflate(trifluoromethanesulfonate), Indium (I) acetate, or Indium(M) acetate.The concentration of the indium salts may range from about 5 to about200 mM, preferably from about 50 to about 150 mM. The concentration ofthiourea may range from about 0.5 to about 2 M, preferably from about0.8 to about 1.2 M. The pH value can be adjusted to between about 0 toabout 5, preferably between about 0 and about 2. Inorganic or organicacids can be used to control the pH value of the plating bath. Suitableacids include sulfuric acid, nitric acid, hydrochloric acid, phosphoricacid, acetic acid, nitrilotriacetic acid, oxalic acid, and formic acid.Suitable chelating agents include citric acid, nitrilotriacetic acid,acetylacetone (acac), ethylenediaminetetraacetic acid (EDTA), alkylamineacetic acid, tartaric acid, polyaciylic acid, polyvinylpyrilidinol(PVP), trioctylamines, TOAB, polyvinyl alcohol (PVA), and/or similarcompounds. The temperature of the plating bath may be between 30° C. and90° C., preferably between 40° C. and 60° C.

An indium electroless plating bath at low pH may be prepared as follows.To an aqueous solution of Indium chloride 100 mM and thiourea 1 Mhydrochloric acid is added. The pH may be adjusted around pH=1. Thetemperature of the solution is raised to about 50° C. The whole platingtime may range from 5 to 120 minutes depending on the desired thicknessof Indium layer over copper nanoparticles.

Excess indium salts may be minimized by control of the starting indiumsalt concentration so that only enough indium is needed for certainthickness of indium coating. If a fast deposition rate is required inthe plating process, excess indium salt in an indium plating bath mightnot be avoidable. In this case, an ultrafiltration technique can be usedto separate Cu/In nanoparticles from the In salt solution. Thesuspension of Cu/In nanoparticles can be directly used as coating mediafor a CIGS absorber layer, or coated with a second and/or a third shelllayer of Al and/or Ga as described below, then used as a coating mediafor a solar cell absorber layer. Alternatively, the suspension can bediluted or concentrated to form a nanoparticle ink, paint, or pasteformulation with a desired viscosity and/or dispersivity. Techniquessuch as centrifugation and filtration can also be used to collect coatednanoparticles, either singly or in combination. Then the coatednanoparticles can be redispersed in a dispersion/suspension system toform an ink, paint or paste.

3. Alkaline Aluminum Electroless Plating Bath

In a third example an electroless aluminum plating bath under alkalineor weakly alkaline conditions may contain hydrazine or sodiumborohydride as a reducing agent. An aqueous solution of aluminum saltsmay be prepared with aluminum salts that can be aluminum (I) sulfate,aluminum (III) sulfate, aluminum (I) nitrate, aluminum (III) nitrate,aluminum (I) chloride, aluminum (III) chloride, aluminum (I) Iodide,aluminum (III) iodide, aluminum (I) triflate(trifluoromethanesulfonate), aluminum (III) triflate(trifluoromethanesulfonate), aluminum (I) acetate, or aluminum (III)acetate, and/or similar compounds, at a concentration of 1 to 30 mM,preferably 5 to 10 nM. Water used in the plating bath is distilled anddeionized with a resistance more than 1 Mega Ohms, preferably more than10 Mega Ohms. Chelating, capping, complexing agents or stabilizers suchas sodium citrate or potassium citrate or nitrilotriacetic acid,trisodium salt, or ethylenediaminetetraacetic acid, disodium salt,triethanol amine, pyridine, polyvinylpyrilidinol (PVP), trioctylamines,TOAB, and/or similar compounds may be added into the above indium saltsolution. The concentration of the chelating agents or surfactants mayrange from 10 to 100 mM, preferably from 20 to 60 nM. A reducing agent,(such as hydrazine or sodium boro-hydride) is slowly added into themixture and the final concentration of the reducing agent may range from20 to 200 nM, preferably from 50 to 100 mM. The pH value for the finalsolution is adjusted to between 8 and 12, preferably in the range of 9to 11. The temperature of the plating bath is raised to between 30° C.and 90° C., preferably between 40° C. and 60° C. Precise control of theramping rate and the final temperature allow for precise control of thedeposition rate of indium. In addition to temperature, the pH value,concentration of aluminum in the solution, and the plating time alsodetermine the final thickness of the aluminum layer that is deposited.The plating time may vary from 10 to 300 minutes. The thickness of thealuminum layer also depends on the particle size of the copper orcopper-gallium nanopowders. For copper or copper gallium nanopowderswith an average particle size of 20 nm, the aluminum layer may be madeto the range of 10 to 50 nm. The thickness of the aluminum layer isrelated to the residence time of the nanoparticles in the chemical bath.The suspension of coated Cu or CuGa nanoparticles can be directly usedas coating media for a CIGS absorber layer, or coated with a secondand/or a third shell layer of In and/or Ga and/or Al as describedherein, then used as a coating media for a solar cell absorber layer.Alternatively, the suspension can be diluted or concentrated to form ananoparticle ink, paint, or paste formulation with a desired viscosityand/or dispersivity. Techniques such as centrifugation and filtrationcan also be used to collect coated nanoparticles, either singly or incombination. Then the coated nanoparticles can be redispersed in adispersion/suspension system to form an ink, paint or paste.

4. Acidic Aluminum Electroless Plating Bath

In a fourth example an acidic aluminum electroless plating bath mayinclude aluminum salts and thiourea in an aqueous solution with a low pHvalue. Chelating agents may be used to stabilize aluminum ions ifnecessary for a conformal coating on copper nanopowders. Unlike alkalinealuminum electroless plating, a reducing agent is not required. Aluminumsalts can be aluminum (I) sulfate, aluminum (III) sulfate, aluminum (I)nitrate, aluminum (III) nitrate, aluminum (I) chloride, aluminum (III)chloride, aluminum (I) Iodide, aluminum (III) iodide, aluminum (I)triflate (trifluoromethanesulfonate), aluminum (III) triflate(trifluoromethanesulfonate), aluminum (I) acetate, or aluminum (III)acetate, and/or similar compounds. The concentration of the indium saltsmay range from about 5 to about 200 mM, preferably from about 50 toabout 150 mM. The concentration of thiourea may range from about 0.5 toabout 2 M, preferably from about 0.8 to about 1.2 M. The pH value can beadjusted to between about 0 to about 5, preferably between about 0 andabout 2. Inorganic or organic acids can be used to control the pH valueof the plating bath. Suitable acids include sulfuric acid, nitric acid,hydrochloric acid, phosphoric acid, acetic acid, nitrilotriacetic acid,oxalic acid, and formic acid. Suitable chelating agents include citricacid, nitrilotriacetic acid, acetylacetone (acac),ethylenediaminetetraacetic acid (EDTA), alkylamine acetic acid, tartaricacid, polyacrylic acid, polyvinylpyrilidinol (PVP), trioctylamines,TOAB, polyvinyl alcohol (PVA), and/or similar compounds. The temperatureof the plating bath may be between 30° C. and 90° C., preferably between40° C. and 60° C.

An aluminum electroless plating bath at low pH may be prepared asfollows. To an aqueous solution of aluminum chloride 100 mM and thiourea1 M hydrochloric acid is added. The pH may be adjusted around pH=1. Thetemperature of the solution is raised to about 50° C. The whole platingtime may range from 5 to 120 minutes depending on the desired thicknessof aluminum layer over copper nanoparticles.

Excess aluminum salts may be minimized by control of the startingaluminum salt concentration so that only enough aluminum is needed for acertain thickness of aluminum coating. If a fast deposition rate isrequired in the plating process, excess aluminum salt in an aluminumplating bath might not be avoidable. In this case, an ultrafiltrationtechnique can be used to separate Cu/Al nanoparticles from the Al saltsolution. The suspension of Cu/Al nanoparticles can be directly used ascoating media for a CIAS absorber layer, or coated with a second and/ora third shell layer of In and/or Ga as described above, then directlyused as a coating media for the absorber layer of a solar cell.Alternatively, the suspension can be diluted or concentrated to form ananoparticle ink, paint, or paste formulation with a desired viscosityand/or dispersivity. Techniques such as centrifugation and filtrationcan also be used to collect coated nanoparticles, either singly or incombination. Then the coated nanoparticles can be redispersed in adispersion/suspension system to form an ink, paint or paste.

5. Organometallic Synthesis Using-Metalocenes

In a fifth example, copper (and/or copper gallium, and/or or gallium,and/or any combination thereof) nanoparticles can be coated with a shellof indium (and/or gallium, and/or aluminum) by reacting withorganometallic compounds such as metalocenes (e.g.Indium(I)cyclopentadienyl (lnCp).

In the prior art, pure indium particles have been made by ‘homogenousnucleation of particles through the dissolving InCp in toluene and thenadding methanol to initiate homogeneous growth. The prior art has alsotaught that monodispersed metal nanoparticles can be formed bydispersing gold nanoparticles (e.g., 1-100 nm in diameter) in toluenecontaining InCp. By avoiding the addition of methanol to this mixture,homogenous nucleation can be suppressed. Instead heterogeneousnucleation (e.g. for the gold particles) dominates, creating verymonodispersed indium particles with gold in the center of each particle(Yu, H., Gibbons, P. C., Kelton, K. F., and W. F. Buhro. 2001.Heterogeneous Seeded Growth: A Potentially General Synthesis ofMonodisperse Metallic Nanoparticles. J. Am. Chem. Soc. 123: 91989199).

Alternatively, organic nanoparticles between about 1 nm and about 100 nmin diameter may be used in lieu of gold nanoparticles to nucleategrowth, the approach taken by Yu et. al. (2001) can be adapted to formcoated CIG, CIA, CIGA, and/or other similar nanoparticles for use as theactive absorber layer of a solar cell. Alternatively, already existingnanoparticle can be coated with indium (and/or Ga and/or Cu and/or Al)in this way to form nanoparticles of a desired elemental stoichiometry.Organic nanoparticles may be formed by making micelles with asurfactant. At a particular concentration of surfactant, one can obtainspheroids of a desired diameter. Organic nanoparticles may alternativelybe obtained commercially and then coated as described above to form thecore nanoparticles. For example, 40 nm diameter polystyrene beads areavailable from Polysciences Incorporated of Warrington, Pa. In addition,20-nm diameter polystyrene beads are available from Duke Sciences ofPalo Alto, Calif. The organic material in the core nanoparticle, e.g.,polystyrene may be removed by burning out during annealing of the filmto form the absorber layer.

Alternatively, known CVD precursors such as CuCl, copper iodide, orother copper halides, copper diketonates (e.g.Cu(II)-2,2,6,6,-tetramethyl-3,5,-heptanedionate (Cu(thd)2)), Cu(1.1)2,4-pentanedionate, Cu(II) hexafluoroacetylacetonate (Cu(hfac)2),CU(II) acetylacetonate (Cu(acac)4, Cu(II) dimethylaminoethoxide, copperketoesters, other organocopper or organometallic precursors (for examplecontaining Si or Ge)), and/or Cu(OCH(Me)CH?NMe2)), and combinations ormixtures of the above (see J. Phys. IV 2001, 11, 569.) and a galliumprecursor material such as GaCl and/or GaCp can be combined to form CuGananoparticles.

6. Atomic Layer Deposition (ALD)

FIG. 3 illustrates a typical ALD system 300. The system generallyincludes a chamber 302 where a substrate 304 rests on a support 306. Gassources 308 are selectively coupled to the chamber 302 through valves310. Gases may be removed from the chamber 302 by an exhaust system 312.The gas sources 308 typically supply one or more precursor gases A, B,one or more additional reactants R and a purge gas P (typically inertgas, e.g., nitrogen, argon, helium). A suitable heat source, e.g., oneor more infrared lamps, (not shown) adjusts the temperature of thechamber 302 and nanoparticles during ALD.

A typical ALD process involves a sequence Of two different andalternating surface reactions involving two different gaseous reactants.The first reaction exposes the substrate to a pulse of a precursor gascontaining molecules or atoms of interest that are to be deposited. Uponapplication of a pulse of precursor gas, the entire surface within theALD chamber becomes saturated with chemisorbed molecules of theprecursor gas. The atoms of interest attach the precursor gas moleculesto deposition sites on the substrate surface. The second reactionexposes the substrate and attached precursor gas molecules to a pulse ofsecond gas, typically a reducing agent, such as hydrogen, which reactswith the attached precursor gas molecules and removes undesiredcomponents of the precursor gas leaving the atoms of interest attachedto the surface at the deposition sites.

The ALD system is typically purged of reactant gases in between thesereactions with a non-reactive purge gas, such as argon or nitrogen,which serve to remove excess chemical species and by-products from thereaction chamber. The separate and pulsed application of the secondprecursor gas followed by the purge with non-reactive gas ensures thatno gas-phase reactions take place in the gas-phase. Rather, chemicalreactions occur on exposed surfaces within the ALD reaction chamber. Thepreceding sequence may be repeated with the original precursor gas orwith a different precursor gas. Such a technique may readily be appliedto formation of GIGS absorber layers.

ALD thus permits a IB-IIIA-VIA absorber layer (e.g., a CIGS layer) to bebuilt up layer-bylayer, using stepwise deposition of partial atomicmonolayers during each application cycle, with the aggregate growth ratedirectly proportional to the number of reaction cycles rather than thepressure or concentration of precursor gases in the chamber. ALDtechniques can thus deposit thin films on the core nanoparticles oneatomic layer at a time, in a “digital” fashion. Such “digital” build-upof material greatly simplifies thickness control. Thus, coatingthickness, and stoichiometric ratios for the coated nanoparticles, maybe carefully controlled using ALD.

To coat nanoparticles by ALD it is often desirable to fluidize theparticles, i.e., to agitate them in some way so that they areintermittently suspended above the surface within the chamber 302,during which time all the surface area of the suspended particle isaccessible for surface-reactions. According to a particular embodimentof the invention, the support 306 includes a vibration mechanism, e.g.,a piezoelectric crystal coupled to an ultrasonic source 314.Nanoparticles 316 are placed on the substrate 304 (or more typically onan electrode layer (e.g., molybdenum) formed on the substrate 304. Theultrasonic source then causes the support 306 to vibrate propelling thenanoparticles 316 upward. By appropriate adjustment of the frequency,amplitude and duty cycle of the signal from the ultrasonic source 314,the nanoparticles 316 may be effectively fluidized for coating by ALD.

The proper choice of precursor materials A, B is important for the ALDprocess to proceed effectively. Appropriate materials typically exhibitthe following features: (I) sufficient volatility at the reactiontemperatures, thermal stability with minimal or no self-decomposition,significant reactivity with the second precursor (reducing agent), andsubstantial insolubility of both precursors in both the product film andthe underlying substrate. Limited solubility can however be tolerated ifthe out-diffusion of a precursor material is rapid enough to go tocompletion during a short purging period. Limited thermal stability canalso be tolerated if the temperature ranges for the deposition processesare well controlled.

For copper, suitable precursors include but are not limited to Cu(I) andCu(II) compounds such as CuCl, copper iodide, or other copper halides,copper diketonates (e.g. Cu(II)-2,2,6,6,-tetramethyl-3,5,-heptanedionate(Cu(thd)₂)), Cu (II) 2,4-pentanedionate, Cu(II)hexafluoroacetylacetonate (Cu(hfac)₂), Cu(II) acetylacetonate(Cu(acac)₂), Cu(II) dimethylaminoethoxide, copper ketoesters, otherorganocopper or organometallic precursors (for example containing Si orGe)), and combinations or mixtures of the above.

For indium, suitable precursors include but are not limited to indiumchloride, indium iodide, or other indium halides, dimethylindiumchloride, trimethylindium, indium (III) 2,4-pentanedionate (indiumacetylacetonate), indium hexafluoropentanedionate, indiummethoxyethoxidc, indium methyl(trimethylacetyl)acetate, indiumtrifluoropentanedionate, and other organoindium (e.g., indium-containingbeta-diketonates) or organometallic precursors (for example containingSi or Ge), and combinations or mixtures of the above.

For gallium, suitable precursors include but are not limited todiethylgallium chloride, gallium triiodide, or gallium (I) halides suchas gallium chloride, gallium fluoride, or gallium iodide, galliumacetate, Ga (III) 2,4-pentanedion ate, Ga (III) ethoxide, Ga(III)2,2,6,6-tetramethylheptanedionate, tris(dimethylaminogallium), and otherorganogallium (e.g., gallium-containing beta-diketonates) or organometallie precursors (for example containing Si or Ge), and combinations ormixtures of the above. For Ga(III)-based organometallic precursors, thereaction conditions required for proper surface reactions in the ALDchamber are likely to be substantially harsher than those required for aGa(I)-based organometallic precursor.

For aluminum, Suitable precursors include but are not limited toaluminum chloride, aluminum iodide, or other halides, dimethylaluminumchloride, aluminum butoxides, aluminum di-s-butoxide ethylacetoacetate,aluminum diisopropoxide ethylacetoacetate, aluminum ethoxide, aluminumisopropoxide, aluminum hexafluoropentanedionate, Al(III)2,4,-pentanedionate, Al(III) 2,2,6,6-tetramethyl-3,5-heptanedion ate,aluminum trifluoroacetate, trisisobutylaluminum, aluminum silicate, andother organoindium or organometallic precursors (for example containingSi or Ge), and combinations or mixtures of the above.

ALD-based synthesis of coated nanoparticles may also (optionally) use ametal organic precursor containing selenium such as dimethyl selenide,dimethyl diselenide, or diethyl diselenide or a sulfur-containing metalorganic precursor, or H₂Se or H₂S, or other selenium- orsulfur-containing compounds, and combinations or mixtures of the above.

To react any of the above precursor materials on the substrate surface,ALD reactions require an additional reactant R, often a reducing agentor proton-donor compound. This reactant can be introduced concurrentlywith the first (Precursor) reactant (especially if the compounds do notcross-react prior to interacting with one another at the substratesurface), or the additional reactant R can be introduced subsequent tothe introduction of the initial precursor. When an organometallicprecursor is hydrated, a proton-donor compound may not be necessary.

Reducing/proton-donating compounds include but are not limited tohydrogen, water (H₂O), methanol, ethanol, isopropyl alcohol, butylalcohols, and other alcohols, and combinations or mixtures of thesematerials, as well as carbon monoxide (CO). Oxygen gas (0₂) may also beused as the additional reactant, or a mixture of H₂O and H₂O₂. Forcertain precursors, especially hexafluoro-pentanedionate (HFPD)precursors such as copper (II) hexafluoropentanedionate, indiumhexafluoro-pentanedionate, and gallium hexafluoro-pentanedionate,formalin (37% formaldehyde, and 15% methanol in distilled deionizedwater) is often used as the reducing agent while nitrogen gas (N₂) isused as the purge gas.

In some situations, a seed layer, e.g., of platinum or palladium may bedeposited on the substrate before ALD with these precursors.

During the deposition process, a typical ALD cycle consists of 1-2seconds of a first metal organic precursor pulse, followed by a 1-2second purge, 1-2 seconds of a reducing agent (or other reactant) pulse,followed by a 1-2 second purge. In general, the duration of the pulseand/or purge cycles may range from about 0.001 seconds to about 60seconds, more preferably from about 0.01 to about 20 seconds, and mostpreferably from about 0.1 to about 10 seconds. The pulse sequence may berepeated any number of times to produce the desired thickness of metallayer on the core nanoparticles. Subsequent pulse sequences withprecursors of different metals can build up multiple layer coatings in acontrolled fashion.

7. Combinations and Extensions of the Above Techniques

Further, the above techniques can be combined in various ways (e.g.,electroexplosion of CuGa wires to make Copper-gallium nanoparticles,followed by wet chemical techniques to coat indium and/or gallium and/oraluminum and/or copper onto such nanoparticles, followed by seleniumexposure and/or ALD using selenium precursors to coat a selenium layeronto the nanoparticles.

Furthermore, a 1 to 100 nm organic nanoparticle can be coated optionallywith Cu, and/or Ga, and/or In, and/or Se, where the organic core is usedas a highly monodispersed seed particle. Coating of such organic coreparticles can be done with any of the dry and/or wet chemical techniquesdescribed above.

Forming an Absorber Layer

The flow diagram of FIG. 4 illustrates an example of a method forforming an absorber layer for a photovoltaic device using coatednanoparticles. The method begins at step 402 by coating corenanoparticles with one or more layers of metal from group TB, IIIA orVIA in a controlled fashion such that the resulting coated nanoparticleshave a desired stoichiometric ratio of elements and appropriate crystalphase as described above.

At step 404 an ink is formed with the nanoparticles. Generally, an inkmay be formed by dispersing the nanoparticles in a dispersant (e.g., asurfactant or polymer) along with (optionally) some combination of othercomponents commonly used in making inks. Other components include,without limitation, binders, emulsifiers, anti-foaming agents, dryers,solvents, fillers, extenders, thickening agents, film conditioners,anti-oxidants, flow and leveling agents, plasticizers and preservatives.

If particle surfaces attract one another, then flocculation can occur,resulting in aggregation and decreasing dispersion. If particle surfacerepel one another, then stabilization can occur, where fine particles donot aggregate and tend not to settle out of solution. Dispersantsprevent ultrafine flocculating particles from reaggregating. Classes ofdispersants include but are not limited to surfactants and polymers.Surfactants are surface-active agents that lower the surface tension ofthe solvent in which they dissolve. In addition to their role asstabilizers, they also serve as wetting agents, keeping the surfacetension of a medium low so that the ink interacts with the substrate.Surfactants are molecules that contain both a hydrophobic carbon chainand a hydrophilic polar group. The polar group can be non-ionic. If thepolar group is ionic, the charge can be either positive or negative,resulting in cationic or anionic surfactants. Zwitterionic surfactantscontain both positive and negative charges within the same molecule; oneexample is N-n-Dodecyl-N,N-diemthyl betaine. Surfactants are often usedas dispersant agents for aqueous solutions, especially When thesurfactant concentration is above the critical micelle concentration(CMC).

Polymers are naturally occurring or synthetic chains consisting of largemolecules made up of a linked series of repeated simple monomers. Boththe chemical and physical nature of the monomer (size, shape, atomicconstituents, polarity, and the like) and the number of linked monomers(e.g. number of monomer repeats, leading to changes in polymer molecularweight) change the polymer properties. Polymers are often used asdispersant agents for pigments suspended in organic solvents.

Foam can form from the release of various gases during ink synthesis andfrom gas introduced during mixing. Surfactants adsorb on the liquid-airinterface and stabilize it, accelerating foam formation. Anti-foamingagents prevent foaming from being initiated, while defoaming agentsminimize of eliminate previously-formed foam. Anti-foaming agentsinclude hydrophobic solids, fatty oils, and certain surfactants, all ofwhich penetrate the liquid-air interface to slow foam formation.Anti-foaming agents include both silicone and silicone-free materials.silicone-free materials include microcrystalline wax, mineral oil,polymeric materials, and silica- and surfactant-based materials.

Binders and/or resins are used to hold together pigment particles.Examples of typical binders and/or include acrylic monomers (both asmonofunctional diluents and multifunctional reactive agents), acrylicresins (e.g. acrylic polyol, amine synergists, epoxy acrylics, polyesteracrylics, polyether acrylics, styrene/acrylics, urethane acrylics, orvinyl acrylics), alkyd resins (e.g. long-oil, medium-oil, short-oil, ortall oil), adhesion promoters such as polyvinyl pyrrolidone (PVP), amideresins, amino resins (such as melamine-based or urea-based compounds),asphalt/bitumen, butadiene acrylonitriles, cellulosic resins (such ascellulose acetate butyrate (CAB)), cellulose acetate proprionate (CAP),ethyl cellulose (EC), nitrocellulose (NC), or organic cellulose ester),chlorinated rubber, dimer fatty acids, epoxy resin (e.g. acrylates,bisphenol A-based resins, epoxy UV curing resins, esters, phenol andcresol (Novolacs), or phenoxy-based compounds), ethylene co-terpolymerssuch as ethylene acrylic/methacrylic Acid, -E/AA, E/M/AA or ethylenevinyl acetate (EVA), fiuoropolymers, gelatin (e.g. Pluronic F-68),glycol monomers, hydrocarbon resins (e.g. aliphatic, aromatic, orcoumarone-based such as indene), maclic resins, modified urea, naturalrubber, natural resins and gums, rosins, modified phenolic resins,resols, polyamide-polybutadienes (liquid hydroxyl-terminated),polyesters (both saturated and unsaturated), polyolefins, polyurethane(PU) isocynates (e.g. hexamethylene diisocynate (HDI), isophoronediisocyanate cycloaliphatics, diphenylmethane disiocyanate (MDI),toluene diisocynate (TDI), or trimethylhexamethylene diisocynate(TIVID1)), polyurethane (PU) polyols (e.g. caprolactonc, dimer-basedpolyesters, polyester, or polyether), polyurethane (PU) dispersions(PUDs) such those based on polyesters or polyethers, polyurethaneprepolymers (e.g. caprolactone, dimerbased polyesters, polyesters,polyethers, and compounds based on urethane acrylate), Polyurethanethermoplastics (TPU) such as polyester or polyether, silicates (e.g.alkyl-silicates or water-glass based compounds), silicones (aminefunctional, epoxy functional, ethoxy functional, hydroxyl functional,methoxy functional, silanol functional, or cinyl functional), styrenes(e.g. styrene-butadiene emulsions, and styrene/vinyl toluene polymersand copolymers), or vinyl compounds (e.g. polyolefins and polyolefinderivatives, polystyrene and styrene copolymers, or polyvinyl acetate(PVAC)).

Emulsifiers are agents that blend liquids with other liquids bypromoting the breakup of aggregating materials into small particles andtherefore stabilize their suspension in solution. Examples ofemulsifiers are sorbitan esters such as sorbitan sesquioleate (Arlacel60), sorbitan sesquioleate (Arlacel 83), sorbitan monolaurate (Span 20),sorbitan monopalmitate (Span 40), sorbitan monostcarate (Span 60),sorbitan tristearate (Span 65), sorbitan monooleate (Span 80), andsorbitan triolcate (Span 85).

Polymeric emulsifiers include polyoxyethylene monostearate (Mytj 45),polyoxyethylene monostearate (Myt j 49), polyoxyl 40 stearate (Myt j52), polyoxyethylene monolaurate (PEG 400), polyoxyethylene monooleate(PEG 400 monolcatc) and polyoxyethylene monostearate (PEG 400monostearate).

The Tween series of surfactants are polyoxyethylene derivatives ofsorbitan-based surfactant compounds like the Span series products. Tweensurfactants are hydrophilic, generally soluble or dispersible in water,and soluble in varying degrees in organic liquids. They are used foroil-in-water (0/W) emulsification, dispersion or solubilization of oils,and wetting. Frequently, Tween surfactants are combined with similarlynumbered Span surfactants to promote emulsion stability. These materialsinclude polyoxyethylene sorbitan monolaurate (Tween 20), polyoxyethylenesorbitan monolaurate (Tween 21), polyoxyethylene sorbitan monopalmitate(Tween 40), polyoxyethylene sorbitan monostearate (Tween 60),polyoxyethylene sorbitan tristearate (Tween 61), polyoxyethylenesorbitan mono-oleate (Tween 80), polyoxyethylene sorbitan monooleate(Tween 8 1), and polyoxyethylene sorbitan tri-olcate (Tween 85). Myij,Arlacel, Tween and Span are registered trademarks of ICI Americas, Inc.,of Wilmington, Del.

Solvents can be aqueous (water-based) or non-aqueous (organic). Whileenvironmentally friendly, water-based solutions carry the disadvantageof a relatively higher surface tension than organic solvents, making itmore difficult to wet substrates, especially plastic substrates. Toimprove substrate wetting with polymer substrates, surfactants are addedto lower the ink surface tension (while minimizing surfactant-stabilizedfoaming), while the substrate surfaces are modified to enhance theirsurface energy (e.g. by corona treatment).

Typical organic solvents include acetate, acrylates, alcohols (butyl,ethyl, isopropyl, or methyl), aldehydes, benzene, dibromomethane,chloroform, dichloromethane, dichloroethane, trichloroethane, cycliccompounds (e.g. cyclopentanone or cyclohexanone), esters (e.g. butylacetate or ethyl acetate), ethers, glycols (such as ethylene glycol orpropylene glycol) hexane, heptane, aliphatic hydrocarbons, aromatichydrocarbons, ketones (e.g. acetone, methyl ethyl ketone, or methylisobutyl ketone), natural oils, terpenes, terpinol, and/or toluene.

Thickening agents are used to tune and optimize ink viscosity to matchthe requirements of a particular printing process. Thickening agentsmodify the theological properties of the ink (Rheology is the study ofthe relationship between an applied stress and the resulting deformationarising from that stress). Examples of theological behavior includepscudoplasticity, where an ink becomes runnier when stirred or spread,and non-newtonian behavior, where ink viscosity changes when the ink isstirred. The greatest contribution to viscosity of any dispersionconsisting primarily of a binder (resin), solvent(s) and pigment is dueto the particular nature of the pigment. Its particle size is also animportant contributing factor. Usually, the larger the particle size,the lower the surface area and the surface energy, and the less likelyare primary particles to form new assemblies. Consequently, they showlower viscosity and increased flow relative to their small particle sizecounterparts. Rheology Modifiers/thickeners can be associative (wherethe modifier strongly interacts with other ink particles), such ashydrophobically modified alkali-soluble emulsions (HASE),‘hydrophobically modified cellulose (HM EC), hydrophobically modifiedethoxylate urethane (HEUR), styrene maleic annhydride copolymer (SMAc),and carboxylic and polycarboxylic acids. Rheology modifiers can also benon-associative, where the modifier does not interact strongly withother ink components. Exampled of non-associative modifiers includecellulosic materials, such as carboxymethylcelluose (CMC),hydroxyethylcellulose (H EC), methylcellulose (methocel, or MC), methylhydroxychtyl cellulose (MHEC), and methyl hydroxypropyl cellulose(MHPC). Additional examples of typical thickness modifiers includecolloidal silicas, metal organic gellants (e.g. based on eitheraluminate, titanate, or zirconate), natural gums (e.g. alginate,carrageean, guar, and/or xanthan gums), organoclays (e.g. attapulgite,bentonite, hectorite, and montmorrillonitc), organowax (such as castoroil derivatives (HCO-Wax) and/or polyamide-based organowax),polysaccharide derivatives, and starch derivatives. Viscositydepressants can also be added to decrease ink viscosity.

Other thickening agents include mixtures of polymers of variousmolecular weights (“low-solids”, or inks containing relatively littlesolid matter). These more traditional thickening agents (such asnitrocellulose polymers) required large amounts of solvent to dissolveHigh MW polymers. Modern thickening agents include “high-solid”polymers, such as polyacrylate homopolymers and copolymers, where bothmonomeric and oligomeric polymeric precursors can be polymerized in situafter ink deposition, for example upon exposure to UV light or otherenergy sources. Polyactylates are especially useful for water-basedinks. In situ polymerization typically requires initiators that produceradicals when exposed to energy that initiate the polymerizationprocess. Examples of initiators include amine-based photoinitiator andcross-linking/coupling agents such as acetoacetates, amides, amines(including aminoplasts, benzoguanamine, melamine-formaldehyde, andureaformaledhyde), annhydrides, aziridines, carbodiimides, diisocynates,mercapto compounds, silanes, and titanate coupling agents. titanatecoupling agents include isopropyl diolcic(dioctylphosphate) titanate(NDZ-101), isopropyl tri(dioctyl)phosphato titanate (NDZ-102), isopropyltrioleoyl titanate (N DZ-105), isopropyl tri(dodecyl)benzenesulfonyltitanate (NDZ-109), isopropyl tristearoyl titanate (NDZ-130), isopropyltri(dioctyl)pyrophosphato titanate (NDZ-201), di(dioctyl)pyrophosphatoethylene titanate (NDZ-311), the triethylamine adduct of NDZ-31 1(NDZ-31 1), tetraisopropyl di(dioctyl)phosphito titanate (NDZ-401), andtetraisopropyl titanate (TPT).

Thickening agents can be used to achieve a particular blockingresistance, where ink adheres only to its intended substrate, by tuningthe glass transition temperature of the material.

Antioxidants are used to retard deterioration of coating films caused byoxidation or heat exposure, and are typically based on molecules thatwill scavenge free radicals as they are formed. A range of antioxidantsis available, including materials based on phenolic compounds (e.g.primary and secondary amines, lactone, phenolic compounds, phosphite,phosphonite, thioesters, and stearic acids, as well aschemically-related compounds.

Flow and leveling agents reduce the surface tension under either or bothdynamic and static conditions, to obtain an optimal wetting and levelingeffect, and to improve the surface flow of the coating. Poor surfaceflow can induce coating defects such as orange-peel, craters, brushmarks, and other surface defects. Examples of typical flow and levelingagents include cellulose acetobutyrate, fluorosurfactants,polyacrylates, silicone, and any of a variety of waxes, including amidewax, bee's wax, carnauba wax, microcrystalline wax, paraffin wax,polyethylene wax, polypropylene wax, and PFTE wax.

Dispersants, binders, emulsifiers, anti-foaming agents, dryers,solvents, fillers, extenders, thickening agents, film conditioners,anti-oxidants, flow and leveling agents, plasticizers and preservativescan be added in various combinations to improve the film quality andoptimize the coating properties of a nanoparticulate ink, paste, orpaint. The use of these materials in the formulation of a semiconductorink, paste, or paint is not limited to nanoparticulates formed by themethods described above, but also nanoparticles formed through a widevariety of nanoparticles synthesis techniques, including but not limitedto those described, e.g., in Published PCT Application WO 2002/084708 orcommonly assigned U.S. patent application Ser. No. 10/782,017, thedisclosures of both of which are incorporated herein by reference.

At step 406 a thin film of the ink, paint, or paste is then formed on asubstrate (typically having a coating made of a suitable conductivematerial, such as molybdenum), e.g., by any of a variety of coatingmethods including but not limited to contact printing, top feed reverseprinting, bottom feed reverse printing, nozzle feed reverse printing,gravure printing, microgravure printing, reverse microgravure printing,comma direct printing, roller coating, slot die printing, meiyerba.rcoating, lip direct coating, dual lip direct coating, capillary coating,ink hetprinting, jet deposition, spray deposition, and the like. The useof these and related coating and/or printing techniques in thenon-vacuum based deposition of a semiconductor ink, paste, or paint isnot limited to ink, paste, and/or paints formed from nanoparticulatesderived by the methods described above, but also using nanoparticlesformed through a wide variety of other nanoparticles synthesistechniques, including but not limited to those described, e.g., inPublished PCT Application WO 2002/084708 or commonly assigned U.S.patent application Ser. No. 10/782,017.

A film can be deposited on a flexible substrate, in a roll-to-rollmanner (either continuous or segmented) using a commercially availableweb coating system. At step 408, the thin film is annealed, e.g., byrapid thermal processing at about 200° C. to about 600° C. A group VIAelement may be incorporated into the film to form the absorber layerabsorber layer. The group VIA element may be incorporated duringannealing (e.g., by exposure to a sulfur containing vapor such as FLS,or a selenium containing vapor such as Se vapor or Fl—)Se, or both).Further, the group VIA element may be incorporated into thenanoparticles during steps 402 or 404.

Photovoltaic Devices

FIG. 5 depicts an example of a photovoltaic cell 500 that uses acombination of 1B-IIIA-VIA materials as components of an absorber layer.The cell 500 generally includes a substrate or base layer 502, a baseelectrode 504, a IB-IIIA-VIA absorber layer 506, a window layer 508, anda transparent electrode 510. The base layer 502 may be made from a thinflexible material suitable for roll-to-roll processing. By way ofexample, the base layer may be made of a metal foil, such as titanium,aluminum, stainless steel, molybdenum, or a plastic or polymer, such aspolyimides (P1), polyamides, polyetherctherketone (PEEK),Polyethersulfone (PES), polycthcrimide (PEI), polyethylene naphtalate(PEN), Polyester (PET), or a metallized plastic. The base electrode 504is made of an electrically conducive material. By way of example, thebase electrode 504 may be a layer of Al foil, e.g., about 10 microns toabout 100 microns thick. An optional interfacial layer 503 mayfacilitate bonding of the electrode 504 to the substrate 502. Theadhesion can be comprised of a variety of materials, including but notlimited to chromium, vanadium, tungsten, and glass, or compounds such asnitrides, oxides, and/or carbides.

The absorber layer may be fabricated using coated nanoparticles, e.g.,as described above with respect to FIG. 4. By way of example, andwithout limitation, the absorber layer 506 may include material of thegeneral formula CuIni_,Gax(S or Se)₂. The absorber layer 506 may beabout 0.5 micron to about 5 microns thick after annealing, morepreferably from about 0.5 microns to about 2 microns thick afterannealing.

The window layer 508 is typically used as a junction partner for theabsorber layer 506. By way of example, the bandgap adjustment layer mayinclude cadmium sulfide (CdS), zinc sulfide (ZnS), or zinc selenide(ZnSe) or some combination of two or more of these. Layers of thesematerials may be deposited, e.g., by chemical bath deposition, to athickness of about 1 nm to about 500 nm.

The transparent electrode 510 may include a transparent conductive oxidelayer 509, e.g., zinc oxide (ZnO) or aluminum doped zinc oxide (ZnO:A1),or Indium Tin Oxide (ITO), either of which can be deposited using any ofa variety of means including but not limited to sputtering, evaporation,CBD, electroplating, CVD, PVD, ALD, and the like.

Alternatively, the transparent electrode 510 may include a transparentconductive polymeric layer 509, e.g. a transparent layer of doped PEDOT(Poly-3,4-Ethylenedioxythiophene), which can be deposited using spin,sip, or spray coating, and the like. PSS:PEDOT is a doped conductingpolymer based on a heterocyclic thiophene ring bridged by a diether. Awater dispersion of PEDOT doped with poly(styrenesulfonate) (PSS) isavailable from H.C. Starck of Newton, Massachussetts under the tradename of Baytron®P. Baytron® is a registered trademark of BayerAktiengesellschaft (hereinafter Bayer) of Leverkusen, Germany. Inaddition to its conductive properties, PSS:PEDOT can be used as aplanarizing layer, which can improve device performance. A potentialdisadvantage in the use of PEDOT is the acidic character of typicalcoatings, which may serve as a source through which the PEDOT Chemicallymay attack, react with, or otherwise degrade the other materials in thesolar cell. Removal of acidic components in PEDOT can be carried out byanion exchange procedures. Non-acidic PEDOT can be purchasedcommercially. Alternatively, similar materials can be purchased from TDAmaterials of Wheat Ridge, Colo., e.g. Oligotron™ and Aedotron™. Thetransparent electrode 510 may further include a layer of metal (e.g., NiAl or Ag) fingers 511 to reduce the overall sheet resistance.

An optional encapsulant layer (not shown) provides environmentalresistance, e.g., protection against exposure to water or air. Theencapsulant may also absorb UV-light to protect the underlying layers.Examples of suitable encapsulant materials include one or more layers ofpolymers such as THZ, Tefzell® (DuPont), tefdel, thermoplastics,polyimides (PI), polyamides, polyetheretherketone (PEEK),Polyethersulfone (PES), polyetherimide (PEI), polyethylene naphtalate(PEN), Polyester (PET), nanolaminate composites of plastics and glasses(e.g. barrier films such as those described in commonly-assigned,co-pending U.S. patent application Ser. No. 10/698,988, to Brian Sagerand Martin Roscheisen, filed Oct. 31, 25 2003, and entitled“INORGANIC/ORGANIC HYBRID NANOLAMINATE BARRIER FILM”, which isincorporated herein by reference), and combinations of the above.

Embodiments of the present invention provide low-cost, highly tunable,reproducible, and rapid synthesis of a nanoparticulate Cu—In and Cu—Gamaterial for use as an ink, paste, or paint in solution-depositedabsorber layers for solar cells. Coating the nanoparticles allows forprecisely tuned stoichiometry, and/or phase, and/or size, and/or shape,e.g., for Cu/In or Cu/Ga nanoparticles.

While the above is a complete description of the preferred embodiment ofthe present invention, it is possible to use various alternatives,modifications and equivalents. Therefore, the scope of the presentinvention should be determined not with reference to the abovedescription but should, instead, be determined with reference to theappended claims, along with their full scope of equivalents. Theappended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase “means for.”

1. A composition of matter comprising: a powder or solution containingcoated nanoparticles made of core nailoparticles, coated with one ormore layers containing one or more elements of group IB. IIIA or VIA,wherein each core nanoparticle contains an element of group IB, IIIA orVIA, wherein at least one of the one or more layers contains an elementthat is different from one or more of the group IB, IRA or VIA elementsin the core nanoparticle.
 2. The composition of claim 1 wherein the corenanoparticles include copper and/or indium and/or gallium and/oraluminum.
 3. The composition of claim 2 wherein the one or more layersincludes a layer of indium.
 4. The composition of claim 3 wherein theone or more layers includes a layer of gallium.
 5. The composition ofclaim 1 wherein the core nanoparticles include copper and gallium. 6.The composition of claim 5 wherein the one or more layers includes alayer of indium.
 7. The composition of claim 5 wherein the corenanoparticles includes copper core nanoparticles coated with indium andcopper core nanoparticles coated with gallium.
 8. The composition ofclaim 1 wherein the coated nanoparticles are mixed with other componentsto form an ink, paint or paste.
 9. The composition of claim 8 whereinthe other components include one or more dispersants, binders and/orresins, emulsifiers, anti-foaming agents, defoaming agents, dryers,solvents, fillers, extenders, thickening agents, film conditioners,anti-oxidants, flow and leveling agents, plasticizers and/orpreservatives.
 10. A method for coating nanoparticles, comprising thesteps of: placing nanoparticles on a stage; vibrating the stage tofluidize the nanoparticles; and coating the nanoparticles fluidized byvibration of the stage using atomic layer deposition.
 11. Anoptoelectronic device, comprising: an electrode and a transparentelectrode; an absorber layer disposed between the electrode and thetransparent electrode, wherein the absorber layer contains elements ofgroups IB, IIIA and VIB; and a window layer disposed between theabsorber layer and the transparent electrode, wherein the absorber layeris formed by the method of claim
 1. 12. The device of claim 11 whereinthe absorber layer includes copper, indium, and selenium or sulfur. 13.The device of claim 12 wherein the absorber layer further includesgallium.
 14. The device of claim 11 wherein the transparent electrodeincludes a transparent conductive polymeric layer.
 15. The device ofclaim 14 wherein the transparent conductive polymeric layer includespoly-3,4-ethylenedioxythiophene (PEDOT) doped withpoly(styrenesulfonate) (PSS).
 16. The device of claim 11 wherein thetransparent electrode includes a layer of zinc oxide.